Synthetic fibers and thermoplastic short‐fiber‐reinforced polymers: Properties and characterization

Among the synthetic fibers, glass fibers (GF) are most widely used in thermoplastic short‐fiber‐reinforced polymers (SFRP), as they offer good strength and stiffness, impact resistance, chemical resistance, and thermal stability at a low price. Carbon fibers (CF) are applied instead of GF, when highest stiffness is required. Other types of synthetic fibers like aramid (AF), basalt (BF), polyacrylonitrile (PAN‐F), polyethylene terephthalate (PET‐F), or polypropylene fibers (PP‐F) are rarely used in SFRP, although they offer some advantages compared with GF. The aim of this article is, to give an overview of various fiber types with regard to their mechanical properties, densities, and prices as well as the performance of their thermoplastic composites. The mechanical properties are presented as Ashby plots of tensile strength versus tensile modulus, both in absolute and specific (absolute value divided by density) values. This overview also focuses on modification of fiber/matrix interaction, as interfacial adhesion has a huge impact on composite performance. A summary of established methods for characterization of fibers, polymers, and composites completes this article. POLYM. COMPOS., 35:227–236, 2014. © 2013 Society of Plastics Engineers     

Effect of branch frequency on dynamic fatigue behavior of slowly notched polyethylene polymers

Samples with the same weight average molecular weight and molecular weight distribution but different branch frequency were utilized to study the effects of branch frequency and thermal history on tie molecule density and their subsequent influence on the slow crack growth of short chain branched polyethylenes. The dynamic fatigue properties are improved significantly with increasing branch frequency and with samples crystalloid at fast cooling rate. However, at temperatures ranging from -20 to 80 °C. the amount of the failure cycle (Nf) improved due to the slight increase in branch frequency is less than those of samples prepared by crystallization at fast cooling rate. Additionally, it is interesting to note that the drawn fibers observed on the fracture surfaces were larger and longer for samples associated with longer Nf. In fact, it is interesting to note that the average number of tie molecules formed per chain (T(M)) of samples associated with longer Nt is also larger. This increasing in T(M) is suggested to be responsible for the improved fatigue properties of samples associated with larger branch frequency and crystallized at fast cooling rate.     

Effect of nanoparticle size and size‐distribution on mechanical behavior of filled amorphous thermoplastic polymers

Different types of polymer nanocomposites on the base of polystyrene, polymethylmethacrylate, and polycarbonate with alumina and SiO₂ nanoparticles and carbon nanotubes have been studied. Miniaturized, microdimensional samples were used, enabling a good control of morphology and distribution of particles by means of transmission and scanning electron microscopy. Special preparation techniques had been applied, which resulted in a very good dispersion of the nanoparticles. Using these materials with really nanosized particles of a few 10nm in size the effect on toughness enhancement could be studied without agglomerates as they often appear in the generally used large samples. Micromechanical mechanisms were studied in detail by TEM and SEM investigations of deformed samples. A “nanoparticle modulated crazing” could be detected as a toughness enhancing effect. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Rheological and mechanical behavior of long‐polymer‐fiber reinforced thermoplastic pellets

Polymer–polymer materials consist of a thermoplastic matrix and a thermoplastic reinforcement. Recent research activities concentrate on the manufacturing of semi‐finished polymer–polymer materials in other shapes than the commercially available tapes and sheets. In particular, a pellet‐like form provides the possibility of processing the polymer–polymer material by injection and compression molding. Nevertheless, the thermoplastic reinforcement is vulnerable to excessive heat and the processing usually needs special attention. The current study investigates the processing of long‐polymer‐fiber reinforced thermoplastic pellets, namely polypropylene‐polyethylene terephthalate and a single‐polymer polyethylene terephthalate, by extrusion for subsequent compression molding applications. The flow characteristics of the material as well as the preservation of the polymer reinforcement can be handled by accurate temperature control. The tensile and impact properties decrease with increasing process temperature though. Moreover, the results prove that the use of a common long‐fiber reinforced thermoplastic process chain is applicable to the newly developed polymer–polymer material. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39716.     

Curing behavior and properties of benzoxazine‐co‐self‐promoted phthalonitrile polymers

A new high‐performance copolymer was successfully obtained via concerted catalysis polymerization of mono‐functional benzoxazine (P‐a) and self‐promoted 4‐aminophenoxy phthalonitrile (4‐APN) monomers. The FTIR and DSC curves of the P‐a/4‐APN in different blend ratios suggested that the monomer blends can be completely cured without the addition of curing additive. The P‐a/4‐APN copolymers were cured at relatively lower curing temperatures and time. The TGA curves revealed that the P‐a/4‐APN copolymers have good thermal stability in terms of T₅, T₁₀, and char yield. A gradual increase in the glass transition temperature (T_g) values and decline were seen in the storage modulus as the loading of 4‐APN was increased from 10 to 30 wt % in the copolymer. The SEM analyses showed that copolymer's fracture surface is dendritic, showing the stress has been dispersed to a certain extent. The study revealed that the poly(P‐a/4‐APN) copolymer have much better thermal stabilities than the poly(P‐a), and the prepared copolymer can be used as a high‐performance thermosetting resin. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46578.     

The mechanical and fatigue properties of flowable crosslink thermoplastic polymer blends based on self‐catalysis of transesterification

A flowable crosslink polymer blend was successfully developed through a reactive compounding process. An epoxy captained ethylene acrylate copolymer and a carboxylic acid and zinc ion contained ethylene acrylic copolymer were employed to react in a twin screw extruder to form a partially crosslink polymer blend which was flowable at high temperature due to the rapid transesterification catalyzed by the zinc ion in the polymer. The developed crosslink polymer blend showed a significant improvement of the mechanical strength, thermal stability, and fatigue performance compared to the neat ethylene acrylic copolymer because of the strong chemical crosslink among polymer chains. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44964.     

Effect of temperature and frequency in fatigue of polymers

The effects of frequency and temperature on fatigue crack propagation rate in poly(methyl methacrylate) and polycarbonate have been studied using centrally notched plate specimens cycled in tension between constant stress intensity limits. Crack growth was monitored at frequencies between 0.1 Hz and 100 Hz and at temperatures between ?60°C and 40°C. A linear relationship between the cyclic crack growth rate $\text{d}\text{(2a)}\text{d}\text{N}$ and appropriate levels of toughness, K, has been proposed: $\text{d}\text{(2a)}\text{d}\text{N}\text{= A?}{}^{\mathrm{\alpha }}$, where $\text{? =}\text{(λ ? λ}{}_{\mathrm{<mtext>th</mtext>}}\text{)}\text{(K}{}^2{}_{\mathrm{<mtext>1</mtext><mtext>C</mtext>}}\text{? K}{}^2{}_{\mathrm{<mtext>max</mtext>}}\text{)}$, λ = K²_max ? K²_min, λ_th is the threshold limit and A and α are constants. Also, the influence of mean stress intensity was briefly discussed.     

The fracture properties of a fiber metal laminate based on a self‐reinforced thermoplastic composite material

The present research program has studied the fracture properties of a Fiber‐Metal Laminate (FML) system constituted by aluminum alloy and a high‐impact self‐reinforced composite material. Here, the self‐reinforced composite system consists of a polypropylene matrix reinforced with polypropylene fibers. Initial testing has shown that a though adhesion can be achieved between the aluminum layers and the composite material by incorporating a thermoplastic adhesive interlayer at the common interface. The adhesion at the metal–composite interface has been studied under a wide range of strain rate conditions using a Single Cantilever Beam test geometry, and it has been shown that the interfacial fracture toughness is loading rate sensitive. Interlaminar delamination tests of the plain composite have also been studied and it was shown that their fracture toughness is also loading rate sensitive. Additional tensile tests have shown that the tensile strength and moduli of the FMLs are linearly influenced by the volume fraction of their constituent materials as well as are successfully predicted using a simple rule of mixture. Low velocity impact tests have also shown that the FMLs based on a self‐reinforced polypropylene composite yielded specific perforation energies well above the 30 J m²/kg. It was also shown that by increasing the number of metal and composite plies in the FMLs, resulted in hybrid structures capable of absorbing higher specific low velocity impact energies. POLYM. COMPOS., 35:427–434, 2014. © 2013 Society of Plastics Engineers     

Effect of thermal cycling on carbon fiber‐reinforced PPS composites

Calorimetry, coefficient of thermal expansion (CTE), and tensile modulus were recorded to investigate the effect of thermal cycling on polyphenylene sulfides (PPS) carbon fiber composites. Thermal cycling at higher temperatures increased the degree of crystallinity of PPS, as indicated by increasing heat of melting. CTE measurements during thermal cycling were used to study the anisotropy of the composites in directions parallel and transverse to the fiber orientation. It was noted that increasing crystallinity enhanced the tensile modulus of unidirectional composites, while reducing the tensile modulus of quasi‐isotropic composites. The latter reduction may be due to internal damage or interlaminar slippage associated with the residual thermal stresses caused by thermal mismatch between multiply oriented plies. POLYM. COMPOS., 26:713–716, 2005. © 2005 Society of Plastics Engineers     

Electric self‐heating behavior of acetylene carbon black filled high‐density polyethylene composites

The electric self‐heating behavior of carbon black (CB) filled high‐density polyethylene (HDPE) was studied in relation to the time‐dependent current and surface temperature under various voltages and to the voltage‐dependent surface temperature at electric–thermal equilibrium. The resistance increase due to self‐heating restricts the current flow through the sample and thus stabilizes the electric power and the self‐heating temperature to their saturation values, which vary with the voltage. A simple phenomenological model shows that self‐heating at electric‐thermal equilibrium is involved in the initial resistance, the electric field induced positive temperature coefficient (PTC) transition and the heat dissipation. The influences of annealing and irradiation crosslinking on the self‐heating behavior are discussed. Copyright © 2004 Society of Chemical Industry     

Drawing of self‐reinforced cellulose films

Self‐reinforced cellulose films were prepared by incomplete dissolution of commercial microcrystalline cellulose in LiCl/DMAc solvent and subsequent coagulation of regenerated cellulose in the presence of undissolved microcrystalline cellulose. By drawing in wet conditions and subsequent drying, preferred orientation was introduced into the self‐reinforced cellulose films, resulting in significantly improved tensile strength of up to 430 MPa and modulus of elasticity of up to 33 GPa. A linear relationship was observed between applied draw, and the orientation of cellulose in the films, and the measured elastic modulus and tensile strength, respectively. The optically transparent drawn films significantly surpass the strength and modulus of elasticity of current all‐bio‐based planar materials and may therefore present a bio‐degradable alternative to nonbio‐based materials with similar performance. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2703–2708, 2007     

Short‐term flexural creep behavior and model analysis of a glass‐fiber‐reinforced thermoplastic composite leaf spring

Present work investigated the short‐term flexural creep performance of fiber reinforced thermoplastic injection molded leaf springs. Unreinforced polypropylene, 20 wt % short and 20 wt % long glass fiber reinforced polypropylene materials were injection‐molded into constant thickness varying width mono leaf spring. Short‐term flexural creep tests were performed on molded leaf springs at various stress levels with the aid of in‐house developed fixture integrated with the servo‐hydraulic fatigue machine. Spring rate reduction is reported as an index for the accumulated damage. Experimental creep performance of molded leaf springs for 2 h was utilized to predict the creep performance with the aid of four parameter HRZ model and compared with 24‐h experimental creep data. Test results revealed that HRZ model is sufficient enough to predict short‐time flexural creep performance of engineering products over wide range of stress. Test results also confirmed the suitability of long fiber reinforced thermoplastic material for creep application over other considered materials. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Comparative swelling behavior of various thermoplastic polymers

The elasticity of polymer melts is of major concern in the processing of plastics. It is usually reflected by dimensional changes. Since the swelling of polymer extrudates depends on the capillary dimensions and the volumetric flow rate, the blow-up must be examined over a range of conditions. Of course, the swelling is also dependent on polymer structure. Consequently, variations in materials and operating conditions necessiate changes in tooling. This paper describes the swelling behavior of several different polymer types and illustrates that viscosity measurements can not be used to predict elasticity.     

Effect of the consolidation degree on the fracture and failure behavior of self‐reinforced polypropylene composites as assessed by acoustic emission

In this work, the fracture and failure behavior of self‐reinforced polypropylene composites (SRPPC) was studied. As reinforcement woven fabric, whereas as matrix materials α and β crystal forms of isotactic polypropylene (PP) homopolymer and random PP copolymer (with ethylene) were used. Composite sheets were produced by a film‐stacking method and compression molded for constant holding time and at constant pressure but at different processing temperatures to obtain SRPPC sheets with different consolidation quality. The failure behavior of tensile specimens was assessed by the acoustic emission (AE) technique and the typical failure behavior was deduced for the differently consolidated composites. Both the number of AE events and the shape of the cumulative AE events versus deformation curve depend on the adhesion between phases. Correlations between the dominant failure mechanisms and AE events amplitude for model specimens were established which can be used to monitor the damage growth process in SRPPCs. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Effect of self‐nucleation on crystallization and melting behavior of polypropylene and its copolymers

The effect of self‐nucleation on the crystallization and melting behavior of isotactic polypropylene (i‐PP) and low ethylene content propylene–ethylene copolymers were investigated. Isothermal crystallization kinetics were studied using the Avrami equation and Lauritzen‐Hoffman nucleation theory. It was found that self‐nucleation can enhance the crystallization. The surface free energy ς_e decreased for the self‐nucleated sample. The melting behavior was affected by the preselected temperature, T_s, at which the polymer was partially melted. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1559–1564, 1999     

Study on the heating behavior of Ni-embedded thermoplastic polyurethane adhesive film via induction heating

The heating behavior of nanoscopic and microscopic Nickel particle-embedded thermoplastic polyurethane (TPU) adhesive under induction heating is studied. Different particle sizes and content of Nickel were applied to TPU with varying film thicknesses and output power of the induction heater. From the results, heat generation of the TPU films increased with increases in Nickel content, TPU film thickness, and output power. The heat generation of the Nickel particle-embedded TPU films was in the order of 70 nm > 1 µm > 70 µm > 20 µm in terms of particle size, and this result can be explained by increases in the ratio of eddy current heating to hysteresis heating with increases in particle size.     

Tung oil‐based thermosetting polymers for self‐healing applications

Several bio‐renewable thermosetting polymers were successfully prepared from tung oil through cationic polymerization for the use as the healing agent in self‐healing microencapsulated applications. The tung oil triglyceride was blended with its methyl ester, which was produced by saponification followed by esterification. The changes in storage modulus, loss modulus, and glass transition temperature as functions of the methyl ester content were measured using dynamic mechanical analysis. In addition, the fraction of cross‐linked material in the polymer was calculated by Soxhlet extraction, while proton nuclear magnetic resonance, Fourier transform infrared spectroscopy and TEM were used to investigate the structure of the copolymer networks. The thermal stability of the thermosets as a function of their methyl ester blend contents was determined by thermogravimetric analysis. Finally, the adhesive properties of the thermosets were studied using compressive lap shear and the fracture surfaces were analyzed using SEM. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40406.     

Double dynamic self‐healing polymers: supramolecular and covalent dynamic polymers based on the bis‐iminocarbohydrazide motif

Self‐healing polymers are a class of functional polymers that, by the virtue of the presence of certain dynamic chemical linkages, may undergo self‐repair at a mechanically cut surface. Herein we report the synthesis of a self‐healing polymer giving access to double dynamicity within the polymer network by making use simultaneously of reversible covalent bonds and dynamic non‐covalent hydrogen bonding interactions. These features are provided, respectively, by doubly dynamic cassettes comprising chemically reversible imine linkages and multiply hydrogen‐bonded urea groups, connected by a siloxane‐based backbone that imparts softness to the material. Such a system can be envisaged to give access to a broad spectrum of functional materials, which can be tuned by convenient modulation of the structural motifs of the polymer. © 2013 Society of Chemical Industry     

Fabrication of the self‐reinforced composites using co‐extrusion technique

The results of this work relate to the use of co‐extrusion technology in the preparation of monocomposite pellets. The low‐melting polypropylene copolymer was used as a matrix material. The high strength polypropylene fibers were used as a fibrous reinforcement. Research confirms the possibility to produce the pellets with fibrous structure. The prepared composite material in the form of pellets was processed and shaped using the injection molding technology. Obtained samples were subjected to mechanical testing in the static tensile test and dynamic mechanical analysis. Research complements microscopic observation of scanning electron microscopy. The measurement results confirm the reinforcing effect of the fibers. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41180.     

Development of basalt fiber‐reinforced thermoplastic composites and effect of PE‐g‐MA on composites

The thermoplastic matrix composites have gained great importance in last three decades. The chopped basalt fiber (mineral fiber) is considered to be a good fiber due to excellent properties as potential reinforcement of composite materials. In this work, composites of chopped basalt fiber (6 mm) with thermoplastic material Nylon‐6 (Polyamide‐6) were prepared and its mechanical and morphological properties were evaluated for automobile applications. The melt blending was carried out in corotating twin‐screw extruder and injection‐molded test samples were prepared for the analysis. The test samples of composite without coupling agent prepared by varying the loading of basalt fiber content of 5%, 10%, 15%, 20%, and 25% by weight and with coupling agent composite loading of Nylon‐6 and basalt fiber content were kept constant and the coupling agent (PE‐g‐MA) loading were changed as 1, 2, 3, 4, and 5 phr. The Mechanical and SEM properties were evaluated. From the test results, it was observed that the mechanical properties were improved with increasing coupling agent ratio. SEM images show good dispersion and adhesion of matrix and reinforcement. POLYM. COMPOS., 38:2798–2805, 2017. © 2015 Society of Plastics Engineers